Speaker
Description
Tungsten is the leading candidate for plasma-facing components in fusion devices. In future fusion reactors, neutron irradiation creates material defects that trap hydrogen isotopes, including the fusion fuel tritium and deuterium. Monitoring the hydrogen inventory is mandatory for nuclear safety and important for the efficient use of fuel. Laser-based diagnostic methods such as Laser-Induced Breakdown Spectroscopy (LIBS) and Laser-Induced Ablation with detection of volatile species by Quadrupole Mass Spectrometry (LIA-QMS) provide quantitative, depth-resolved assessments of the local fuel inventory and release. They are offering a controlled measurement of near-surface retention on short time scales for in-situ and ex-situ applications.
To describe the effect of transient material heating on fuel release beyond the ablated layer, a one-dimensional modelling framework is developed in FEniCS/FESTIM for picosecond laser pulse trains. The simulations address depths below 1 mm. Transient heat conduction with temperature-dependent properties is coupled to a minimal near-surface trapping-detrapping scheme. Initial near-surface fuel fractions are in the percent to sub-percent range. Coarse ablation is represented by removing a thin surface layer tens of nanometers thick at the start of each pulse, which updates both the thermal and hydrogen-isotope boundary conditions. Per-pulse release is obtained by integrating the simulated desorption flux. Evolution over fuel release is analyzed for tens to hundreds of pulses.
Here in this modelling approach, the initial fuel distribution is taken from NRA depth profiles measured on proton-irradiated and self-damaged, deuterium-decorated tungsten samples which act as reference.
Results show that heating-induced release dominates over the contribution attributable to the imposed ablation layer release. Early pulses are characterized by rapid mobilization of the near-surface fuel reservoir, leading to pronounced, prompt desorption transients. Consequently, when ablation is applied at the start of subsequent pulses, it largely removes a layer already partially depleted by prior heating. With a simple defect-annealing term, model predictions are compared to LIA-QMS-derived depth reconstructions, yielding reasonable agreement.
The modelling framework supports the interpretation of LIA-QMS data for self- or proton damaged tungsten as proxy for neutron-damaged tungest and clarifies the respective roles of fast heating and material ablation in pulse-train experiments with ps-laser systems.
Work supported by BMFTR project SyrVBreTT.